Automotive Fittings - Getting Fit

Here's the scenario: It's Saturday morning and you begin a project/update/repair on your street rod that you've told yourself will only take half an hour or so to complete. Before you know it, an hour has passed and you're still searching through several coffee cans full of fittings for the last one needed to complete the chore, but nothing seems to fit. In desperation, you jam a couple of fittings together with enough Teflon tape to bandage your head, but your rod now leaks like the Valdez. So you're off to the local parts house where the kid behind the counter wants to know the make, model, and year of the car, and if it has air conditioning. He then points you to a cabinet full of fittings, most of which are in the wrong bins, and now the challenge is to figure out what is compatible with what, so you waste even more time trying to make things screw together. If something like this has ever happened to you, your pals at STREET RODDER want to help.

When it comes to automotive fittings, the possible combinations will melt your brain, but thanks to the help of Gary Clausen and Jason Hofman of Pure Choice Motorsports, we're going to explain the most common fittings and show what they look like.

Pipe ThreadsThere are actually several types of pipe threads, but the most common in automotive use is the American Standard Pipe Taper Thread, or NPT. Unlike the threads on a screw, or other fittings for that matter, pipe threads are tapered. Because of the taper, a pipe can only screw into a fitting a certain distance before it begins to jam.

When dealing with pipe fittings, figuring out the proper size can be confusing. At one time, size was determined by the pipe's internal diameter-a misconception that continues today, even among some of those who sell the stuff for a living. The correct method to determine pipe size for cars is now determined by the Society of Automotive Engineers' guidelines for Dryseal American Standard Tapered Pipe Threads (NPTF). With this system, pipe size does not agree with either the inside diamter (id) or ouside diameter (od), or thread pitch. The proper means to determine pipe size (up to 1 1/4-inch) is to measure the diameter of the threads and subtract 1/4-inch. For example, subtract 1/4-inch from a 1-inch thread od to obtain the nominal pipe size of 3/4-inch.

While NPTF fittings are supposed to make a leak-proof mechanical seal, some sort of sealer-such as Teflon tape or Teflon-based paste-is usually advisable. When using either, use caution so as not to get either inside the assembly. The best way to do this is to stay off the first two threads.

Inverted FlareOne of the most common types of fittings found on automobiles is the SAE double inverted flare. It is unique in that the flare is doubled, or folded back on itself, for increased strength and resistance to cracking. The tube flare is clamped between the nut and flare seat of body when screwed together so when the assembly is tightened, a leak-proof metal-to-metal joint is created and no sealer is necessary.

Typical inverted flare applications are brakes, power steering, fuel, and transmission cooler lines. These connections are vibration-resistant and when used with the proper tubing will withstand working pressure up to 2,000 psi with burst pressures up to 5,000 psi.

SAE 45-Degree FlareFound on instruments, power steering, and some hydraulic (not brakes) and fuel lines, these differ from inverted design in that a single flare is used. Care should be used during installation as over tightening can cause the tubing to split.

Compression FittingsIntended for low- and medium-pressure applications, compression fittings are easy to assemble because they do not require flaring or soldering. A sleeve is used over the tubing, and when the nut is tightened, the wedging action of sleeve between body and nut create a tight, leak-free connection.

Often used on instruments, and some air and hydraulic lines, compression fittings are used on a variety of tubing materials; however, they should never be used on brakes.

Brake-Fitting AdaptersHydraulic brake-fitting adapters can be used to connect brake lines to different size ports in a dual master cylinder, combine different types of fitting-such as inverted flare, pipe, and AN-or to mix standard and metric fittings.

AN FittingsConventional automotive lines and fittings use 45-degree double flares; AN lines and fittings use 37-degree single flares. The reference "AN" stands for Army/Navy and it was a system devised by the government to ensure interchangeability and compatibility of parts made by various manufacturers. With the AN system, a dash number is assigned to each different size metal tubing and the corresponding fittings, and the same numbers are also assigned to the hose and their ends. Another application of 37-degree flares will be found on JIC hydraulic fittings.

AN fittings are often found on street rods in combination with pipe and inverted flare fittings, which means special adapters are required. Suppliers, such as Pure Choice Motorsports, can supply such fittings as well as lines, individual parts, or complete kits.

AdaptersLook long and hard enough and you're likely to find an adapter to connect just about any two fittings together. However, keep in mind that when it comes to brakes, only 45-degree double-flare or 37-degree AN components should be used.

Clamp-Style Hose FittingsClamp-style hose fittings are manufactured for low-pressure applications with multipurpose hose (worm-style clamps are good for a maximum of 35 psi). A hose clamp is required between the raised end of the fitting and the end of the hose.

Push-On Hose FittingsPush-on hose fittings are manufactured for low and medium pressure (maximum pressure 200 to 250 psi). These fittings are quick connecting and have high resistance to vibration damage. When used in combination with the proper hose, no clamps are required.

Teflon TapeIn most cases, properly flared and installed fittings should seal by themselves; however, Teflon tape or Teflon-based sealing compound is often used on pipe threads.

Teflon tape can prevent leaks and make it easier to tighten fittings and disassemble them later, and it can reduce/eliminate thread galling and protect fasteners made from corrosion. However, there are some places it should never be used, such as the engine's electrical senders and the cooler line fittings in automatic transmissions.

Used on senders, it will insulate them, preventing the proper grounding necessary for accurate gauge operation. On transmission lines, it can make the pipe fittings tighten so easily that the tapered threads can split the case.

So Many ChoicesIf we included every fitting available in the form of tubing and fitting, we wouldn't have room for anything else in the magazine this month, so we've tried to cover the most common items a street rodder will come across. Next time we'll show how to properly flare tubing, use sealants, and build solid and rigid lines, and a bunch more. In the meantime, stay fit!